A noise canceling microphone system is gaining more importance nowadays, especially with the development of multimedia applications and wireless communication technologies. Although various solutions were proposed to enhance desired signal extraction, particularly desired speech, in noisy environments, there is still room for improvement in order to obtain a high Signal-to-Noise (SNR) ratio using very few microphones.
Various methods were commonly used to increase the SNR of desired speech signal. In a known speech enhancement method, a single microphone is used to pick up the desired speech signal with noise. The noise spectrum is estimated and subtracted from the speech signal (containing the noise) picked up by the microphone. In this way, the desired speech signal is separated from the noise. However this method is only effective with stationary noise, and also introduces high distortion to the desired speech signal.
Another known noise cancellation method uses two microphones, with one microphone located near the source of the desired signal, and another microphone located near the noise source. Thus, the signal picked up by the microphone arranged near the noise source can be used to adaptively cancel the noise signal from the signal picked up by the microphone arranged near the desired speech signal. This method is not practical in most applications as it is very difficult to arrange a microphone near the noise source.
A further known microphone array system uses more than two microphones. The system uses a spatial and temporal filtering method to enhance the desired speech signal from a specific direction and over an interested frequency band, and suppress signals from other directions. The desired signal obtained with this system has a high SNR. However the use of more than two microphones results in a large system, unsuitable for use in many mobile applications.
In the system disclosed in U.S. Pat. No. 4,742,548, more than three microphones are used to form a unidirectional microphone system for noise cancellation. Since no adaptive signal processing method is used in this system, the spatial response of this microphone system is fixed. This makes the whole system inflexible.
According to the system described in U.S. Pat. No. 5,226,076, two microphones are used to form a first-order microphone system for noise cancellation. However, this microphone system uses only the differential property of sound field to form a fixed beam pattern, the performance of the system is therefore poor, especially in environments with complicated noise signals.
system disclosed in WO 0,195,666, a cardioid-type directional microphone and an omni-directional microphone are combined in an acoustically coupled way. The two microphones, together with an adaptive control circuit, produce a very narrow 3-dimensional beam for acquiring the desired speech signal. However, the direction of the 3-dimension beam in this system is fixed. In other words, when the desired signal is coming from another direction which is different from the direction the beam is projected, the microphone system is not able to acquire the desired signal. The physical orientation of the system needs to be adjusted such that the desired signal falls into the direction of the 3-dimensional beam in order for the desired signal to be acquired. This makes the use of the system inconvenient.
Therefore, a noise cancellation microphone system with a high SNR and which is convenient to use is desired.